A Scheme for Selecting Chairs for Clerical Operations

ABSTRACT

Biomechanical, kinesiological, and subjective assessments of seating were made as part of a chair selection program. Subjects sat for 2 hours sessions at a VDT in each of 8 chairs, during which periodic measurements of joint angles, seat pressures, back IEMGs, and discomfort were acquired. A weighting scheme was developed to combine the various results so that chairs could be matched to the users.

INTRODUCTION

Many vendors attach the label "ergonomic" to their office chairs without performing any research into the human factors involved in chair usage, simply as a marketing ploy. The purpose of this pilot study was to subject a variety of chairs to biomechanical, physiological, and subjective testing to objectively determine which chairs perform acceptably to be considered "ergonomic" and not pose a risk of discomfort to workers in clerical positions.

METHODS

Two subjects volunteered and gave their informed consent to participate in this pilot study: a fifth percentile female (1.49 m, 45 kg) and a 95th %ile male (1.88 m, 102 kg). These subjects were recruited so that we could test the chairs with the range of anthropometries that designers typically aim for. Sagittal plane body positions were obtained by taking video records of reflective markers placed on the following joint centers: knee, hip, shoulder, elbow, wrist, and a marker on the head just in front of the external ear canal.

The electrical activity of four muscle groups was acquired using surface electrodes, and the resulting signals were amplified and processed to yield integrated electromyograms (IEMGs). The muscle groups studied were the erector spinae at L5/S1 (low back), the erector spinae at T2 (upper back), the trapezius muscle (back of the neck), and the medial deltoid muscle (the shoulder). The distribution of pressure on the seat pan surface only (not the chair back) was obtained with a prototype device (Tekscan, Inc., Boston, MA). A mylar sheet housing an array of 1600 photo-resistive ink sensors of one square centimeter each was placed over the seat pan of each of the chairs. This sensor sheet was then attached to a cuff that went to a custom data acquisition board in the laboratory microcomputer. The sensor readings were sampled at 50 Hz for 10 seconds for each trial collected.

Three types of subjective measures were taken. At the end of the study the subjects were asked to rank the chairs based on comfort over the 2 hour sessions. Within each 2 hour session the subjects were queried about relative discomfort of different body parts using a body pictograph. Finally, the chairs were rated for ease of adjustment and range of adjustability by the experimenters setting up the chairs for each session.

Each subject sat in each of the 8 chairs used in this study for two hours without a break. Both objective (position, EMG, and seat pressure) and subjective (body pictograph and overall discomfort level) data were collected at 20 minute intervals throughout the 2 hours, starting before the clock was begun and hence yielding 7 sets of readings for the 2 hours. Two trials were collected for each of the objective measures at each of these 20 minute intervals so that a total of 14 trials were obtained of joint angles, lower and upper back, neck, and shoulder EMGs, and seat pressure patterns. Throughout the 2 hour sessions the subjects sat at a workstation that had a Macintosh computer and associated keyboard. The subjects typed in text, played computer games, or rested their hands on the keyboards while watching television.

Four chairs without arms and four chairs with arms were investigated. The
particular tasks that our subjects were performing did not allow them to use the arms - no analysis of the effects of the arms was made.

RESULTS

The body position information of relevance to this study included the trunk angle and the head angle. Trunk angle is probably the single most important measure since the subjects were allowed to perform various tasks which would have yielded different head angles.

The muscles relevant to chair analysis were upper and lower back erector spinae
and the trapezius (neck) muscles. Data were ranked from the chair with the lowest EMG activity levels to the highest, with the rationale that the chairs requiring the least muscle activity were less likely to cause fatigue or muscle stiffness or soreness due to sustained contraction.

The seat pressure data were post-processed to derive histograms of the number of
pixels (or the area) within each decade of raw pressure values, yielding a quantitative description of the pressure distribution. Three criteria were subsequently applied to these histograms: whether the distribution for a particular chair changed over the 2 hour exposure, the highest pressure sensed between the seat and the subject's posterior, and the amount of area of contact at minimal pressure. The rationales for these criteria were: a chair that shows a change of pressure distribution over time may be giving evidence of cumulative cushion compression which may cause problems for the heavier individual. The highest pressure that the sensor detected usually came under the subject's ischial tuberosities; the lower this maximum pressure the less likely that a stress concentration can reach uncomfortable or harmful levels. Finally, as the amount of surface area that the
subject contacts the seat surface at minimal pressure increases, the better the overall pressure distribution is because it is more spread out.

The body pictograph results were of minimal interest because the subjects rarely
developed any discomfort worth reporting in only 2 hours. There were isolated exceptions to this, however. The subjects did select different chairs as being most comfortable for them.

A weighting scheme was developed to assess which chair performed best for each
of these subjects. The scheme is based on ranking chair performance along the different criteria and assigning points to reward the best ranked chairs. Chairs were ranked from best (3) to not good (0) for each criteria. The weighting factors were assigned with 10=extremely important and relevant to this study, 5=important and somewhat relevant to this study, and 0=unimportant and irrelevant. Total scores were derived by multiplying the chair ranking by the weighting factor for that criterion measure and then summing these weighted scores across all criteria. Notice that the subjective measures are weighted more highly than the objective measures; this compensates for the fact that there are 8 objective but only 3 subjective criteria. The seat pressures were weighted higher than the joint angles
and the EMGs because of their direct bearing on the discomfort. Tables 1 and 2 present the results for the 5th %ile female and the 95th %ile male respectively:

Table 1: Weighted chair scores for the 5th %ile female.

CRITERION MEASURE

Weight

CH 1

CH 2

CH 3

CH 4

CH 5

CH 6

CH 7

CH 8

Joint Angles

Trunk nearest 90 degrees

5

3(15)

2(10)

1(5)

0

0

0

0

0

Min. change in trunk angle

5

3(15)

0

1(5)

0

0

0

2(10)

0

Muscle Activity (EMGs)

Min. erector spinae (L5)

5

0

0

2(10)

3(15)

0

1(5)

0

0

Min. erector spinae (T2)

5

0

3(15)

0

2(10)

0

1(5)

0

0

Min. trapezius

3

0

3(9)

2(6)

1(3)

0

0

0

0

Seat Pressures

Min. change over time *

7

0

2(14)

0

0

2(14)

2(14)

0

2(14)

Lowest maximum pressure

9

0

0

1(9)

0

0

1(9)

3(27)

2(18)

Greatest area at low press.

6

0

2(12)

1(6)

0

0

1(6)

3(18)

1(6)

Subjective Measures

Min. total discomfort

10

1(10)

0

0

2(20)

0

0

3(30)

3(30)

Subject preference

8

2(16)

0

0

1(8)

3(24)

0

0

0

Ease of adustment

10

3(10)

0

2(20)

0

3(30)

2(20)

1(10)

0

TOTALS

0

80

46

75

56

54

61

95

68

* all of these chairs showed no change over time

Table 2: Weighted chair scores for the 95th %ile male.

CRITERION MEASURE

Weight

CH 1

CH 2

CH 3

CH 4

CH 5

CH 6

CH 7

CH 8

Joint Angles

Trunk nearest 90 degrees

5

3(15)

0

1(5)

0

0

0

2(10)

0

Min. change in trunk angle

5

2(10)

1(5)

0

0

3(15)

0

0

Muscle Activity (EMGs)

Min. erector spinae (L5)

5

0

0

0

0

3(15)

1(5)

0

2(10)

Min. erector spinae (T2)

5

2(10)

0

3(15)

0

0

0

0

1(5)

Min. trapezius

3

2(6)

0

0

0

3(9)

0

0

1(3)

Seat Pressures

Min. change over time *

7

0

0

3(21)

0

0

0

0

0

Lowest maximum pressure

9

0

0

2(18)

0

2(18)

2(18)

2(18)

3(27)

Greatest area at low press.

62(12)

1(6)

0

1(6)

0

3(18)

1(6)

Subjective Measures

Min. total discomfort

10

1(10)

0

0

2(20)

0

0

3(30)

3(30)

Subject preference

8

0

0

3(24)

0

2(16)

0

0

0

Ease of adustment

10

3(30)

0

2(20)

0

3(30)

2(20)

1(10)

0

TOTALS

0

97

17

139

0

109

53

56

61

* all of these chairs showed no change over time

CONCLUSIONS

There are several limitations to this study that need to be reiterated so that the results can be kept in their proper perspective. First, only two subjects participated; however, they each spent 16 hours in chairs so it was not as if they were naive. Our results show that different chairs should be supplied to the short, light individuals than to the tall, heavy individuals.

Finally, we did not directly address the chair needs of the extremely heavy person
or the person with a known back problem. There are special chairs made for the heaviest individuals. For people with back problems the chairs selected by our subjects should be sufficient because they offer adjustability of back support. We did not test several chairs provided to us that did not allow for independent adjustment of the back and the seat, although some of these chairs may perform quite well for average size individuals.

Let us now put these results into the perspective of the clerical operations at an
actual client. The estimate we were given was that 80% of the clerical stations serve multiple shifts. Our results show that different chairs should be supplied to the short, light individuals than to the tall, heavy individuals. To make this work with minimum chair "migration" between shifts, workers should be assigned to workstations to be shared with other workers of similar height and weight. This will also require a minimum of adjustment for each individual.

The remaining issues with respect to a successful chair replacement program
involve its implementation. Whether an entire department should receive chairs at one time or each department should receive some chairs to feel included has to be decided, and training in chair use has to be provided.